In renal failure patients, normally urine volumes decrease due to deterioration of renal function, often resulting in overhydration. A treatment is therefore required that would pass the blood of the patient through an extracorporeal circulation so that the condition of their body's water can be as nearly normalized as possible. This process of removing water from the body is referred to as “body fluid removal.” Because the total body fluid variation amount is managed based on the removal weight of body fluid during treatment, the removal weight of body fluid is considered the most important parameter in patient management.
In recent years, for the treatment of renal failure or multiple organ failure with serious complications in the circulatory system, the generically called continuous blood purification has proved effective in the areas of emergency and intensive care. The continuous blood purification includes continuous hemofiltration (to be hereafter referred to simply as CHF), continuous hemodialysis (to be hereafter referred to simply as CHD), and continuous hemodiafiltration (to be hereafter referred to simply as CHDF).
CHF is a technique whereby blood is caused to flow in a blood purifying apparatus accommodating a semipermeable membrane in order to expel water containing waste products through the filtering membrane while delivering replacement fluid to the body continuously and slowly. CHD is a technique whereby dialysis for achieving an acid-base equilibrium by osmosis, for example, is performed continuously and slowly. CHDF is a technique combining CHF and CHD, whereby, in order to improve the small molecular-weight removal performance of CHF, a dialysate is caused to flow on the filtrate side of the blood purifying device so that a dialysis effect can be obtained. In any of these blood purifying methods, the continuous and slow treatment is characterized in that, as the name suggests, a single treatment is conducted over several days and blood purification is carried out slowly. Such a treatment greatly differs from the simple hemodialysis or hemofiltration in terms of temporal magnitude, the latter techniques requiring 4 to 5 hours for a single treatment.
A preferable example of a blood purifying apparatus based on the aforementioned continuous blood purification is disclosed in JP Patent Publication (Kokai) No. 9-239024 A. The apparatus comprises at least either a means for feeding a dialysate for hemodialysis or a means for feeding a replacement fluid for hemofiltration, a drain means, and a blood circulation path. Each of the means is equipped with a reservoir container, a fluid transfer pump, and a plurality of weightmeters for weighing the reservoir container. Based on the information provided by each of the weightmeters, the flow rate of each fluid transfer pump is individually controlled. Another example suitable for CHF or CHD is disclosed in JP Patent Publication (Kokai) No. 4-70909 A. The example comprises at least either a means for feeding a dialysate for hemodialysis or a means for feeding a replacement fluid for hemofiltration, a drain means, and a blood circulation path. Each of the means is equipped with a reservoir container and a fluid transfer pump, the reservoir container being provided with a fluid level sensor for detecting an upper limit and a lower limit of the stored quantity of the reservoir container. The apparatus further comprises a single weightmeter for weighing two reservoir containers all at the same time. Based on the information provided by the weightmeter, the flow rate of fluid transfer pump is individually controlled.
FIG. 3 shows the concept of the blood purifying apparatus of the aforementioned first example, which is based on the continuous blood purification. A blood purifying apparatus 50′ is comprised of a blood drawing line 3 and a blood retransfusing line 4, which together form a blood circulation path; a drain line 23 for draining water containing waste products; a replacement fluid line 25 connected to the blood retransfusing line 4 for delivery of replacement fluid to the patient; and a dialysate feeding line 24 for feeding a dialysate to the filtrate side within the blood purifying device 2. In the blood drawing line 3, there is provided a blood pump 1. A blood purifying device 2, which includes a filtration membrane M, is disposed between the blood drawing line 3 and the blood retransfusing line 4.
The drain line 23 includes a drain transfer pump 5 for draining a filtrate and dialysis drain fluid from the blood purifying device 2, a drain reservoir container 8 connected to a drain branch line 17 that branches off on the outlet side of the drain transfer pump 5; and a shutoff valve 14 attached to the drain line 23 downstream of the branch portion. The drain reservoir container 8 is equipped with a weightmeter 26 for drainage weighing purposes.
The dialysate line 24 includes a transfer pump 6 for feeding a dialysate to the filtrate side within the blood purifying device 2; a dialysate reservoir container 9 connected to a dialysate branch line 18 that branches off on the inlet side of the dialysate transfer pump 6; and a shutoff valve 15 attached to the dialysate transfer line 24 upstream of the branch portion. The dialysate reservoir container 9 is equipped with a weightmeter 27 for dialysate weighing purposes.
The replacement fluid line 25 includes a transfer pump 7 for feeding a replacement fluid to the patient; a replacement fluid reservoir container 10 connected to a replacement branch line 19 branching off on the inlet side of the replacement fluid transfer pump 7; and a shutoff valve 16 attached to the replacement fluid line 25 upstream of the branch portion. The replacement fluid reservoir container 10 is equipped with a weightmeter 28 for weighing the replacement fluid.
The blood taken out from a patient using the blood pump 1 passes through the blood drawing line 3 and is then introduced into the blood purifying device 2 including the filtration membrane M, where waste products or the like are removed. In the blood purifying device 2, where a dialysate is supplied by the transfer pump 6 for dialysate, an acid-base equilibrium, for example, is established, and the filtrate and dialysis drain fluid are drained by the drain transfer pump 5. The blood that has been subjected to filtration and dialysis in the blood purifying device 2 is then returned to the patient via the blood retransfusing line 4, in the course of which a replacement fluid of substantially an equal weight to that of the filtrate is added by the replacement fluid transfer pump 7, thus delivering the replacement fluid to the patient.
The device thus does not require frequent weighing or adjustment operations by the staff, and is capable of continuously performing treatment in a safe manner while appropriately controlling the body fluid weight of the patient. Furthermore, a dialysate reservoir unit 21 or a replacement fluid reservoir unit 22 can be exchanged as needed, or, in the case where the filtrate and dialysis drain fluid are collected in a tank, the tank can be exchanged as needed, without directly affecting the measurement of the weight of removed body fluid or without terminating the treatment.
The transfer pumps are associated with certain amounts of flow rate errors. In order to minimize the influence of such errors, in the above-described apparatus, the reservoir containers 8, 9, and 10 are equipped with the weightmeters 26, 27, and 28, respectively, so that data can be supplied from the individual weightmeters to a control unit, which is not shown. The control unit monitors the data from the weightmeters 26, 27, and 28 at all times, and calculates the actual flow rate based on a change in weight per unit time. If it finds a difference between the actual flow rate and a set flow rate, the control unit automatically adjusts the rotation speed of a motor in each of the transfer pumps 5, 6, and 7 individually, such that the set flow rate equals the actual flow rate so as to maintain a flow rate accuracy.
Although the above-described apparatus is capable of maintaining a high flow rate accuracy, it inevitably suffers from errors on the order of 1% in the flow rate accuracy in each transfer pump in actual operations, due to factors such as the temperature characteristics of the weight sensors and of the measurement electronic circuitry, variations with time, methods of adjustment during manufacture, variations in the shape of each reservoir container, and so on.
As described above, the weight of body fluid removed from a renal failure patient ΔV(L), which is managed as an important parameter, is determined by the following equation:ΔV=VF−VC−VD  (1)where VF (L) is the amount of fluid drained by the drain transfer pump 5, VC (L) is the amount of replacement fluid supplied by the replacement fluid transfer pump 7, and VD (L) is the weight of dialysate supplied by the dialysate feed pump 6.
Conventionally, when performing a treatment based on CHDF, the flow rate of the transfer pumps is generally on the order of 1 L/hr. For example, if the flow rate of the drain transfer pump 5 is set at 1 L/hr, that of the replacement fluid transfer pump 7 at 0.5 L/hr, and that of the dialysate transfer pump 6 at 0.5 L/hr, then VF=24±0.24 (L), VC=12±0.12 (L), and VD=12±0.12 (L) in 24 hours, assuming that each transfer pump has a flow rate error of approximately 1%. In this case, if the removal weight of body fluid ΔV is calculated according to equation (1), ΔV=0±0.48 (L), thus indicating that the body fluid removal error can be reduced to approximately 0.48 (L) or less, which corresponds to 2% of the drained volume VF.